The one obvious explanation for the increasing obesity rate is that more people are consuming more calories than they are using, i.e., their energy consumption is greater than their energy expenditure.

This is simple enough. What is not simple are the reasons — Why this phenomenon has happened so quickly and to such a surprising extent.

The top 4 causes of obesity fall into two general categories: genetics and environment.

The current epidemic is almost certainly a combination of the two.

Genetics

The fat in a person’s body is stored in fat cells distributed throughout the body. A normal person has between 25 and 35 billion fat cells, but this number can increase in times of excessive weight gain, to as many as 100 to 150 billion cells.

The number of fat cells in the body remains constant after their formation; the cells just expand and shrink in size during weight gain and loss. This has been suggested as one reason weight loss is so hard to maintain for many people, and research is under way to determine methods or medications that will reduce the actual number of cells.

Four critical periods have been identified during which time the number of fat cells a person has will increase: between 12 and 18 months of age; between 12 and 16 years of age, especially in females (in fact, the best single predictor of adult obesity is adolescent obesity); in adulthood when an individual gains in excess of 60% of their healthy weight, and during pregnancy.

Recent studies of some extremely obese people have discovered a genetic basis for their obesity.

1. In one study, the obese subjects were found to have a genetic defect in the gene coding for a hormone called leptin, which is involved in appetite regulation.

2. In another, the subject was lacking an enzyme, PCI, the function of which is to convert another hormone, PMOC, into neuropeptides that regulate appetite. Without converting PMOC, the subject’s body could not respond to the leptin.

While these findings are intriguing and offer small glimpses into the potential for genetic research into obesity, however, experts say that these genetic defects are rare and are not found in the vast majority of persons at risk from obesity in the U.S. today.

This is not to say that the susceptibility for weight gain is not genetic. There are many ways in which genes affect our weight, from our resting metabolic rate, to how we burn calories when we exercise, to how quickly our brains signal us that we are full.

Age and gender significantly affect metabolic rate. As persons age, their metabolism normally slows down.

Men generally have higher resting metabolic rates than women; women’s rates slow down even more dramatically after menopause.

In a study conducted by the National Institute of Diabetes and Digestive and Kidney Diseases, the resting metabolic rates of over 500 volunteers were analyzed and found to range from burning as few as 1,067 calories a day to as many as 3,015 calories.

Studies of persons adopted at birth have shown no relationship between the adult weights of adoptees and their adoptive parents; instead, the weights of the adoptees more closely resembled, first, the weights of their biological mothers and, secondly, that of their biological fathers. In fact, if a biologic mother is heavy as an adult, there is a 75% chance that her children will be heavy.

In a British study by Parsons et al. maternal weight was found to account for the positive relation noted by many researchers between a larger birthweight and excess weight in adulthood.

Genes also play a role in how our bodies react to exercise. Researchers at Laval University in Quebec found a wide variation among young men in a four-month-long program studying the effects of exercise as measured in several ways, including maximal oxygen uptake, heart size, and muscle fiber size.

Obesogenic Environment

This all suggests that, while one person might be born with a stronger tendency to gain weight than another, the circumstances must be right for this to happen. Genetics play a role, but the gene pool in America has not changed significantly in the few decades during which obesity has become so prevalent.

Genetics must be combined with an environment conducive to gaining weight, an environment that has been termed “obesogenic”, in order for the explosive increase in both childhood and adult overweight and obesity to have occurred.

The evolution of our “obesogenic” environment has been both rapid and multifactorial.

There has been a tremendous increase in the availability of food, especially high-fat and/or high-calorie food, at the same time that there has been a decrease in the amount of individual physical activity.

Researcher James O. Hill of the University of Colorado has observed that, if the obesity epidemic in America is not checked, almost every American will be overweight within just a few more generations. To quote Dr. Hill, “Becoming obese is a normal response to the American environment”.

Changing Food Consumption

The home-cooked family dinner is no longer the norm it once was. The increase in one-person households, single-parent families, and families with two working parents has fueled the demand for easily accessible, inexpensive take-out meals. Of the 30 fastest growing franchises in the United States in 1999, 12 were fast food companies.

According to Dr. David Hunnicutt, president of the Wellness Councils of America, part of McDonald’s corporate mission is the goal of establishing enough outlets that Americans are never more than 4.5 minutes from the nearest franchise.

In 1970, food eaten away from home accounted for 34% of the average American’s food budget; by the late 1990s, this had grown to 47% .
Snacking has become a way of life in the United States; in 1999, Americans consumed 54.5 gallons of soft drinks, 10.3 pounds of chocolate, and 21.4 pounds of chips, pretzels, and nuts per capita.

Not only have our opportunities to find food away from home increased, so have the portion sizes of those foods. In fact, large sizes are now the focus of advertising to lure customers in, whether in the form of supersized meals, 32-, 48-, or even 64-ounce soft drinks, or all-you-can-eat buffets.

Dr. Hunnicutt observes that McDonald’s original meal of a burger, fries, and a 12-ounce Coke had 590 calories; today, a quarter-pounder with cheese, super-sized fries, and a super-sized Coke packs a walloping 1,550 calories, three-quarters of the 2,000 calories a day recommended for many adult women for weight maintenance.

Even some “diet” meals are now advertised as larger sized , the more the better, it seems, regardless of the consequences.

The additional attraction of the fast food industry is the low cost of many of the items, making this high-fat, calorie-dense diet available to just about everyone. Families can eat out for a reasonable price, teenagers can fit in an extra meal after school, and many workers can afford lunch out on a daily basis. Availability, large portions, and low cost present a combination of factors that many Americans can’t resist.

Medications

Antidepressants and Corticosteroids can also result in overweight and obesity.

Physical Inactivity

At the same time our diets have taken a turn for the worse, the amount of physical activity in our lives has decreased. A number of studies have suggested that the increasing prevalence of obesity is in fact more strongly related to decreased energy expenditure than to increased energy consumption. It is clear that an active lifestyle decreases the risk of several chronic diseases and certain physical and mental disabilities, contributes to more efficient functioning of many of our body’s systems, and improves our overall quality of life.

The positive benefits of regular physical activity are present throughout life, during childhood, adolescence, and adulthood. None of these benefits is more important than the critical role physical activity plays in weight maintenance throughout the life span.

Childhood and Adolescence

The best way to avoid obesity is to become active in childhood and then maintain an active lifestyle.

Regular exercise in childhood and adolescence provides many benefits, including building strong bones and muscles, improving strength and endurance, increasing self-esteem and reducing stress and depression, and conferring some of the same positive effects as with adults, e.g., lower blood pressure and cholesterol levels.

Adolescence, in particular, is seen as a critical period for physical activity for several reasons: excess weight in adolescence is a risk factor for adult obesity; physical activity during these years is more likely to be sustained into adulthood; and adolescence is a key time for the development of such risk factors as the onset of coronary artery disease as well as the years of peak development of bone mineral density. Unfortunately, these are also the years when levels of activity decline markedly, especially among females.

A 1993 study concluded that during the school-age years daily physical activity decreases an average of 2.7% among males and 7.4% among females. The daily energy expenditure (relative to body size) of the average 18-year-old is only half of what is was when he or she was 6 years old .

On the positive side, leisure-time physical activity at the age of 16 among males decreased by one-half the risk of being sedentary at age 34.

At least three periods of strenuous exercise a week are recommended for children by health experts, but one in every five (20%) children aged eight through 16 in the United States today fails to achieve this.

According to data provided by the CDC through the School Health Policies and Programs Study 2000, only 71.4% of the states (including the District of Columbia) provided regularly scheduled recess for students in grades kindergarten through five; 82.4% required physical education, but just 8.0% of elementary schools provided daily physical education or its equivalent (150 minutes per week).

Among middle/junior high schools, 84.3% of states required students to take physical education, with 6.4% of schools providing it on a daily basis (or 225 minutes per week).

Eighty-six (86.3%) of states required students to take physical education in senior high school, with 5.8% providing daily classes (or 225 minutes per week).

Watching television is now the number one leisure-time activity among America’s school-age children.

A study by researchers at Johns Hopkins University School of Medicine, in collaboration with the CDC, concluded that over one-fourth (26%) of U.S. children watch four or more hours of television a day, and these hours don’t include time spent playing video games or sitting at a computer, additional sedentary leisure-time activities.

The average high school graduate will have spent approximately 15,000 to 18,000 hours in front of a television set but only about 12,000 hours in school.

The contribution of TV viewing to childhood obesity is twofold: energy expenditure is decreased through the sedentary nature of the pastime and energy consumption is increased by either eating during viewing or responding to advertisements of high-carbohydrate, high-fat foods aimed at children.

Adulthood

The percentage of adults who are sedentary increases approximately twofold between the ages of 20 and 65.

The Behavioral Risk Factor Surveillance System (BRFSS)1 data from 2000 show a direct association between age and physical inactivity.

Only 18.0% of U.S. adults aged 18-24 reported no participation in any physical activity during the month preceding their interview, compared with 34.6% of those aged 65 and older.

As an individual approaches middle age, weight gains can be particularly hazardous.

Many of the chronic illnesses that frequently emerge in middle adulthood have a strong relationship with excess weight, such as type 2 diabetes, cardiovascular disease, osteoarthritis, and hypertension.

The benefits of regular physical activity among older Americans are tremendous.

The World Health Organization addressed these benefits in guidelines published in 1997.

Among them are:

• Improved muscle strength and endurance
• Balance and coordination, and flexibility,
• Lower blood pressure and blood lipids,
• Iimproved cardiovascular fitness,
• Psychological benefits in terms of better cognitive functioning and overall feeling of well-being.

Fewer adults of all ages, however, are expending energy at a rate commensurate with weight maintenance, with our increasing rates of obesity as a result. Just as our increased consumption of fast food has a multifactorial basis, so does our decrease in daily physical activity.

Physical activity has decreased in both work- and leisure-related activities, and labor-saving devices abound in our society.

Work-Related Physical Activity. Work no longer provides the opportunity for physical activity that it once did for many Americans. A century ago in the United States there were 11,553,000 farmers; now there are about 851,000.

At the same time that jobs are becoming more sedentary, the work week has been expanding, allowing even less time for leisure-time physical activity.

According to research presented by Dr. Hunnicutt of the Wellness Councils of America, the average worker in the United States now works the equivalent of one extra month per year compared to workers in 1970.

Leisure-time Physical Activity

Even as leisure-time activity has become more vital, given our sedentary jobs, the percentage of the adult population reporting such activity has not changed significantly over the past decade.

National BRFSS data from 1990 show 71.3% of respondents engaged in some type of leisure-time physical activity; by 2000, this had only risen to 73.1%, meaning that over one-fourth (26.9%) of all adult Americans reported no activity at all.

The CDC and the Surgeon General recommend that all sedentary adults should accumulate at least 30 minutes of at least moderate-intensity physical activity over the course of most, preferably all, days of the week.

Experts agree that to be beneficial, activity does not need to be of high intensity or done all in one session.

An example of a “moderate intensity” activity could be moderate to brisk walking, i.e., at a pace of 15-20 minutes per mile, for the recommended 30 minutes. The 30 minutes can be further divided into three walks of 10 minutes each and still meet the recommendation.

The emphasis is now on “lifestyle physical activity” that which can be maintained throughout an individual’s lifetime.

Without question, a sedentary lifestyle increases the risk of several chronic diseases, depression, a loss of physical functioning, and even premature mortality, and, as previously noted, contributes substantially to the risk for obesity.

While a review of studies examining the link between physical activity and weight loss concludes that exercise does not significantly increase initial weight loss over and above that obtained with diet alone, physical activity has been determined to be essential in the prevention of weight gain.

In addition to this benefit, however, physical activity has been found to provide a protective effect on the health risks associated with obesity, conferring health benefits independent of weight changes.

In a recent review of studies examining fitness and health outcomes of all-cause mortality, heart disease, type 2 diabetes, hypertension, and cancer, the researchers concluded, “active obese individuals actually have lower morbidity and mortality than normal weight individuals who are sedentary”